The present invention relates to a method for lowering the frictional coefficient of the magnetic head-disk interface in a magnetic disk storage system comprising magnetic disks with lubricated surfaces and magnetic heads that are mounted on sliders which in operation of the system are supported on air cushions formed between air bearing surfaces of the sliders and the rotating disk surfaces. The air bearing surfaces of the magnetic head sliders are in contact with the disk surfaces during sliding phases of start/stop operations of the system.
Disk packs containing several lubricated disks and associated head/slider combinations are usually contained in an enclosure. These so-called head-disk assemblies, HDAs, include a closed air system in which the circulating air is filtered so that practically dust-free air is ventilated. Such disk storage systems, HDAs and air systems are described in U.S. Pat. Nos. 4,034,411, 4,190,411, 4,285,018, IBM Technical Disclosure Bulletins Vol. 21, No. 6, November 1978, p. 2514, and Vol. 22, No. 6, November 1979, p. 2474.
Rigid disk files used for on-line storage have a disk pack spinning at high speed. Depending on the application the file is designed for, the spinning speed is from approximately 2,000 to 6,000 rpm, resulting in a 14-inch disk with a linear velocity of 33 m/sec to 100 m/sec, respectively. At 3,600 rpm the linear velocity is 60 m/sec. This is 100 to 500 times faster than the tape speed in a conventional cartridge. Contact of the head with the disk would be a disastrous event at such velocity. Therefore, contact of the head with the disk must be avoided during flight and therefore lubrication of the disk is provided, because of the required proximity of the head to the recording medium. The head is carried on the tail of a slider that is supported on an air bearing, the air bearing being created by an air bearing surface on the slider. Examples of conventional head/slider combinations are shown in U.S. Pat. Nos. 3,823,416; 3,855,625; and 4,251,841.
In a disk file that is not operating, the slider, with its magnetic transducer head on its tail, sits on the disk. When the disk starts to rotate the slider glides in contact with the disk surface until the take-off speed is reached. It will not reach stable flight immediately but contacts the disk several times, causing hopping. Similar dynamic interaction of the slider will happen upon landing when the system is turned off. To minimize friction during these start/stop operations the disk is provided with a lubricant.
Lubrication is generally done by applying oil to the disk. Such oil layer must be thin enough not to impede the flight by accumulation, not to mix with dust or debris, and not to cause lubricant transfer in a large amount to the slider and head. It must be easily applicable and remain on the disk surface over the lifetime of the system. Thus the lubricant must wet the recording medium or be chemically attached and must not form droplets (autophobia) and be spun off. Furthermore, it must not change viscosity, nor cause the slider to stick to the disk at its rest position. If stiction occurs between the slider and the disk the retaining forces can be strong enough to cause permanent distortion and damage of the head suspension. Such incidence can also cause the slider to damage the magnetic layer and generate debris. Due to the air circulation within the head-disk assembly the debris can be transferred to other disks and may obstruct proper flying of the head or cause a head crash.
These described interactions of the slider with a head and the surface of the disk are the most frequently occurring incidents that make a disk file inoperable.
In contrast, there are extremely few incidents that render a disk file unusable due to other malfunctions of the magnetic heads or disks themselves, e.g. for environmental circumstances. Almost all incidents occur as a consequence of the tribological system comprising the slider-head, lubricant, and magnetic medium under the extreme forces and circumstances mentioned above. It has been found out that lubrication does not solve all problems to increase the lifetime of a disk storage system or diminish wear. The head-disk interface problems, such as stiction, i.e. the slider sticking in its rest position to the disk, friction during starting and stopping, depletion of lubricant, or debris on the slider, all lead to possible head crashes and to diminishing disk file life expectancy. One of the causes of these problems appears to be humidity and its influence on the lubricant.
In an article entitled "Stiction at The Winchester Head-Disk Interface" by Chin C. Liu and Peter B. Mee, published in IEEE Transactions on Magnetics, Vol. MAG-19, No. 5, September 1983, pp. 1659-61, the role of humidity in these problems is described. In accordance with this article stiction was found experimentally to be very sensitive to the humidity of the environment. The texture of the disk surface, the slider loading, lubricant thickness, water adsorption, susceptibility of the disk surface, the purging process and water-lubricant interaction were all considered as important parameters for stiction initiation. It was found that a water drop can diffuse through the lubricant layer and displace the lubricant from the disk surface by lateral advance. During the drying process, the surface tension of the thin water film will pull the two solid surfaces together and produce deformation at the areas of actual contact. The thinner the film thickness, the higher the adhesion force which will be produced. The magnitude of stiction thus depends on the amount of water condensation, the degree of water diffusion through the lubricant films and the amount of lubricant replaced by water molecules at the contact area. In the case of severe stiction, a solid-solid junction could be formed.